1. Field of the Invention
The present invention relates to a data communication apparatus and, more particularly, to a data communication apparatus which can be connected to a digital communication line.
2. Related Background Art
An ISDN (Integrated Service Digital Network) is known as a conventional digital communication network.
The ISDN is a digital communication network for offering various types of service such as telephone service, data service, and facsimile communication service. An interface between each user and the network is recommended by the CCITT.
A standard arrangement of an ISDN subscriber is shown in FIG. 3. A TE (ISDN terminal equipment) is connected to an NT.sub.1 (network terminal) and an interface according to CCITT recommendation I.430, so that the NT.sub.1 is connected to the ISDN. A voice from a telephone or image data from a facsimile machine is communicated as a 64-Kbps digital signal via the B channel. Signal information such as link control and call control data are communicated as a 16-Kbps signal via the D channel.
A G3 facsimile machine is a communicating means which is most popular these days. By using a public telephone network, image data is sent using a modem having a transmission speed of 4,800 bps (V27 ter) or 9,600 bps (V29). A procedure signal is sent via a modem of 300 bps (V21).
FIG. 4 shows a general arrangement of a G3 facsimile machine.
Referring to FIG. 4, the G3 facsimile machine includes a reader 1 for reading a "send" original (i.e., an original to be sent), a compression circuit (COMPRESSION) 2 for compressing the read data from the reader 1 by an MH or MR coding scheme, a "SEND PROCEDURE SIG" or procedure signal transmitter 3 for generating a procedure signal of the G3 facsimile machine, a control unit (MPU) 4 for controlling the overall operations of the facsimile machine, a "RECEIVE PROCEDURE SIG" or procedure signal receiver 5 for analyzing a received G3 facsimile procedure signal, a data expansion circuit (EXPANSION) 6 for expanding the received image data, and a printer 7 for printing an output from the expansion circuit 6. The facsimile machine also includes a modem and NCU 8 for modulating/demodulating image data and the procedure signal and performing network control, and a terminal 9 connected to a public telephone network.
Original data read by the reader 1 is compressed by the compression circuit 2. The procedure signal generated by the procedure signal transmitter 3 is modulated by a low-speed modem in the modem and NCU 8 and is then sent onto a communication line. A procedure signal received from the communication line is demodulated by the low-speed modem in the modem and NCU 8, and the demodulated procedure signal is decoded by the procedure signal receiver 5. The image data compressed by the compression circuit 2 is modulated by a high-speed modem in the modem and NCU 8. The modulated image data is sent onto the communication line. Image data received from the communication line is demodulated by the high-speed modem, and the modulated image data is expanded by the expansion circuit 6. The expanded data is printed at the printer 7.
This G3 facsimile machine performs communication via an analog communication line and cannot be used in a digital communication network.
In order to connect the above G3 facsimile machine to a digital communication line, an analog signal output from the modem in the G3 facsimile machine can be converted into a digital signal, while a digital signal from the digital communication network can be converted into an analog signal which is then input to the modem in the G3 facsimile machine.
In this case, however, in the process of A/D-converting a signal D/A-converted by the modem in the G3 facsimile machine again, quantization errors tend to occur, and the circuit arrangement is undesirably complicated.
A facsimile machine having a G3/G4 function is disclosed in U.S. patent application Ser. No. 120,104 filed on Nov. 13, 1987 and a G3 facsimile ISDN adaptor is disclosed in U.S. patent application Ser. No. 192,542 filed on May 11, 1988.